The present invention relates to an image forming apparatus, and particularly to an image forming apparatus wherein pulse width modulation signals corresponding to image signals are obtained and a light source is controlled in accordance with the pulse width modulation signals.
Heretofore, in an image forming apparatus of a type mentioned above, when images are formed employing a laser beam printer based on digitized image signals, digital signals have been converted to analog signals, and the converted signals have been compared with periodical pattern signals (reference wave) such as a triangular wave to generate pulse width modulation signals in order to obtain gradation properties. Then, a laser beam (light source) has been controlled by the pulse width modulation signals generated so that images may be formed on a photoreceptor by means of a laser beam. (see Japanese Patent Publication Open to Public Inspection (hereinafter, referred to as Japanese Patent O.P.I. Publication) No. 183663/1987.)
FIG. 12 shows a conventional modulation circuit for forming pulse width modulation signals.
Here, dot clock DCK synchronized with pixel data DATA is supplied to integrator 22 composed of variable resistor 22a and capacitor 22b through buffer 21. Output signals from the integrator 22 are sent to comparator 26 through a series circuit including resistor 23, buffer 24 and capacitor 25 for cutting DC as pattern signals Sp (reference wave).
The amplitude of the pattern signals Sp is adjusted by the variable resistor 22a so that the overall pattern signals Sp is contained in the full scale (00H to FFH in terms of 8 bits) of D/A converter 28 described later, and an off-set value (DC value) is adjusted by means of variable resistor 27.
In addition, pixel data DATA is sent to D/A converter 28 to be converted to an analog signal and then, it is sent to the comparator 26 as image signal Sv. CLK is a clock for D/A converting.
In the comparator 26, pattern signal Sp sent from the integrator 22 and image signal Sv sent from the D/A converter 28 are compared. Then, from this comparator 26, pulse width modulation signals SPWM based on pixel data DATA is outputted.
In the constitution, when the dot clock DCK is one as shown in FIG. 13A, pattern signal with triangular wave Sp is supplied to the comparator 26 as shown by the solid line in FIG. 13B. Accordingly, when image signal Sv is shown by a dot line in FIG. 13B, pulse width modulation signal SPWM having the same frequency as that of dot clock is outputted from the comparator 26 as shown in FIG. 13C.
Incidentally, remarkable distortion sometimes occurred on the wave form of the dot clock DCK employed for forming the pattern signal Sp in the manner, due to a standing wave generated during transmission or a noise from outside. Thereby there was a fear that the pulse width modulation signal SPWM could not be formed correctly, resulting in deterioration in the reproducibility of gradation of reproduced image.
In order to prevent generating of distortion in the dot clock DCK, it is considered to remove duty ratio change by demultiplying the dot clock DCK.
The solid line in FIG. 13D shows a pattern signal Sp formed by demultiplying the dot clock DCK into two. By the use of the pattern signal Sp, pulse width modulation signal SPWM having the frequency which is double that of the dot clock can be obtained as shown in FIG. 13E.
Since the dot clock DCK is demultiplied to be used, the pulse width modulation signal SPWM having the frequency which is double that the dot clock is not affected by the duty ratio change of the dot clock. Therefore, deterioration in gradation reproduction on a reproduced image can be prevented. In addition, a constitution wherein the pulse width modulation signal SPWM having the same frequency as the frequency of the dot clock is generated has an advantage to obtain high resolution by securing sampling number.
Accordingly, heretofore, in images wherein resolution of a character image is considered important, it was generally conducted to change the frequency of a reference wave, i.e., the ratio of the frequency of an image signal to that of pattern signal (the reference wave) was set to 1:1 for images such as character images whose resolution is important, and the ratio was arranged 1:2 in the case of photographic image wherein gradation reproducibility was considered important.
However, even in the case of changing the frequency in accordance with images mentioned above for use, there was a problem that sampling number was reduced and resolution was deteriorated when the frequency of a reference wave was set to be double that of image signal for forming accurate pulse width modulation signal SPWM. To the contrary, when resolution was considered to be important, there was a problem that gradation reproducibility was reduced by being influenced by duty ratio change of the dot clock as mentioned above. Thus, resolution and gradation reproducibility were contradictory to each other.